Subducting plate structure and megathrust morphology from deep seismic imaging linked to earthquake rupture segmentation at Cascadia

The origin of rupture segmentation along subduction zone megathrusts and linkages to the structural evolution of the subduction zone are poorly understood. Here, regional-scale seismic imaging of the Cascadia margin is used to characterize the megathrust spanning ~900 km from Vancouver Island to the California border, across the seismogenic zone to a few tens of kilometers from the coast. Discrete domains in lower plate geometry and sediment underthrusting are identified, not evident in prior regional plate models, which align with changes in lithology and structure of the upper plate and interpreted paleo-rupture patches. Strike-slip faults in the lower plate associated with oblique subduction mark boundaries between regions of distinct lower plate geometry. Their formation may be linked to changes in upper plate structure across long-lived upper plate faults. The Juan de Fuca plate is fragmenting within the seismogenic zone at Cascadia as the young plate bends beneath the heterogeneous upper plate resulting in structural domains that coincide with paleo-rupture segmentation.

Figure S1.Regional topography with experiment tracklines and line names indicated.Location of industry wells on the shelf (39) shown with colored circles indicating nature of oldest material recovered; red (Eocene basalt), yellow (Lower Eocene arkosic wacke), green (Upper Oligocene and younger mélange).Mid-ocean ridges and Blanco transform fault location shown in light dashed line.Lower plate crustal age and location of propagator shear zones interpreted from offset magnetic isochrons (grey shading) from (29); note that crustal age interpretation and propagator shear zones under the upper slope and shelf are inferred in (29) from magnetic isochrons on conjugate Pacific plate and are shown in fainter colored polygons to indicate lower confidence.Shelf features HB-Heceta Bank; NB-Nehalem Bank.Other annotation as in Figure 1.    4 shown in black-white color scheme.Bottom panel is same section without AGC applied and displayed in red-white-black color scheme.Horizon depths at line crossings are indicated with colored circles and colored arrows point to horizons interpreted in Fig. 4. Other annotations and labels as in Fig. 4.
Note that seismic reflectivity in the region of the interpreted NN2 fault is complex with ambiguous TOC and Moho horizons evident in the seismic images.The step up in TOC across the NN2 fault is identified primarily based on the shift in the depth of the band of higher reflectivity interpreted to mark the oceanic crust, which south of 46.5°N is bounded by clear bright TOC and Moho reflections and which is best illustrated in the seismic image without AGC.The weak discontinuous events interpreted as TOC from ~46.25-47.8°Nare from the top of this band of reflectivity.
Step-up in basement depth at the projected NN2 fault is also required by TOC depths that are well defined on the other dip and strike lines in this region and is supported by other prior seismic imaging which indicates a several kilometer difference in plate depth under the shelf north and south of 47°N (79).
Note mismatch in Moho depth at projected PD13 line crossing (line terminates within data gap in PS01 ~2 km to west).This mismatch could reflect imaging artifacts due to the break in PS01 transect at this location or could reflect crustal thickening associated with nearby seamounts inferred in (48).

Figure
Figure S2.(A) Gridded surface showing depth to top of crust derived from CASIE21 and Ridgeto-Trench PSDM seismic sections along tracklines superimposed on regional bathymetry.Apex of buried seamounts identified from seismic data indicated with small (incoming plate) and large (under accretionary wedge) black stars.(B) Gridded surface showing depth to interpreted plate interface along tracklines and along Daisy Bank fault derived from (56) as described in Methods.White polygon with grey transparent fill shows approximate outline of interpreted regional-scale décollements within sediment column and highlight primary regions where plate interface deviates from top crust surface.Three pairs of white circles indicate crossing of faults in lower plate discussed in text.Deformation front is shown in bold black line; 200 m contour defining the shelf edge indicated in thin black line.Other annotations including strike-slip faults and upper plate backstops discussed in text are as in Fig. 2.

Figure S3 .Figure S4 .
Figure S3.Velocity error analysis from Kirchhoff pre-stack depth migration.(A) Final velocity model overlaid on the final depth stack for line PD12.Blue dotted line is the interpreted top of crust; the location common midpoint (CMP) gather in (B) in thick black line.(B) Results from Kirchhoff pre-stack depth migration showing CMP gather 56000 using three different velocities: the final velocity model shown in A), +5% of the final crustal velocity model, and -5% of the final crustal velocity model.The flatten migrated CMP gathers using the final velocity model flatten the top crust basement horizon while the ±5% velocity models over or under migrate the data, indicating this Vp range overestimates the sensitivity of the velocity model.(C) The interpreted top of crust (blue line) and interpretations depth converted using the ±5% velocity models (red lines) give the range of possible depths for the top of crust of ~±50 m on the incoming plate to ~±900 m under the shelf.

Figure S5 .
Figure S5.Along shelf seismic transect PS01 shown without horizon interpretations and displayed with and without AGC.Top panel corresponds to image in Fig.4shown in black-white color scheme.Bottom panel is same section without AGC applied and displayed in red-white-black color scheme.Horizon depths at line crossings are indicated with colored circles and colored arrows point to horizons interpreted in Fig.4.Other annotations and labels as in Fig.4.

Figure S6 .
Figure S6.Seismic images from Figure 5 shown without horizon interpretations.Horizon depths at line crossings are indicated with colored circles and colored arrows point to some of the interpreted horizons from Fig. 5.Other annotations as in Figure 4.

Figure S7 .
Figure S7.Seismic images from Figure 6 shown without horizon interpretations except for primary wedge faults which are shown in thin red line to aid recognition of sediment décollement horizon.Horizon depths at line crossings are indicated with colored circles and colored arrows point to horizons interpreted in Fig. 6.Annotations as in Figure 6.

Figure S8 .
Figure S8.Close up of portion of seismic transect PD05 offshore Vancouver Island showing frontal thrusts reaching to TOC.Further downdip they shoal to a shallower horizon interpreted as plate interface décollement which merges with TOC at ~ 10 km from DF.Note change in TOC reflectivity character below sediment décollement as seen in S8. (A) With interpretations.(B) Without interpretations.Horizon depths at line crossings are indicated with colored circles and colored arrows point to some of the reflections interpreted in panel A.

Figure S9 .
Figure S9.Seismic image of portion of seismic transect PS06 offshore Vancouver Island showing sediment décollement interpreted to extend through part of this region to ~48.25°N (km 85).TOC beneath sediment décollement is a brighter and lower frequency (wider in depth section) event compared with where sediment décollement is not identified, suggesting presence of a package of subducting lower Vp and higher attenuation (perhaps fluid rich) sediment above.The sediment décollement terminates at a likely fault bounded basement high with fault plane reflections evident which transect the oceanic crust extending into the mantle.Within the sediment column above this lower plate fault, a fault plane reflection extends through the sediment section.(A) With interpretations.(B) Without interpretations.Horizon depths at line crossings are indicated with colored circles and colored arrows point to reflections interpreted in panel A.

Figure S10 .
Figure S10.Close up of portion of seismic transect PD06 offshore Washington showing horizon interpreted as plate interface fault within deep sediments merging with top crust at ~ 38 km from DF.Note change in TOC reflectivity character below sediment décollement as seen in S8 and S9.(A) With interpretations.(B) Without interpretations.Horizon depths at line crossings are indicated with colored circles.

Figure S11 .
Figure S11.Comparison of new plate interface geometry and Slab2 regional plate model (27).(A) Slab depth from Slab2 with tracks lines for current study superimposed.(B) Difference between Slab2 model and plate interface depths from new study also shown in Figure 8. Positive values (red to purple) indicate new plate model is shallower than Slab2; negative values (blues) indicate new model is deeper.

Figure S12 .
Figure S12.Comparison of new plate interface geometry and prior regional plate model of (24).(A) McCrory Slab depth from (24) with tracks lines for current study superimposed.(B) Difference between McCrory model and plate interface depths from new study.Positive values (red to purple) indicate new plate model is shallower than McCrory model; negative values (blues) indicate new model is deeper.

Figure S13 .
Figure S13.Comparison of geometry of plate interface with additional rupture history and slip behavior indicators.(A) Dip of plate interface from Fig 7B and 9 with additional features including the maximum extent of modelled slip patches for the 1700 CE earthquake of (22) in purple semitransparent polygons; location of Outer Arc High from (17) in white.(B) Seismicity catalog from (20) for comparison.Seismicity is classified as from plate interface (red), lower plate (green), upper plate (blue), and incoming plate (black) using Slab2 depth model.Purple/white line shows the interpreted downdip extent of the seismogenic zone based on detected plate interface microseismicity.Thin black line corresponds with 200m bathymetric contour roughly defining the shelf edge.Other annotations as in Figure 7B.